gas mileage compromises
 

Hi,
I have a 2005 single cab with a Smarty, a stock turbo over a s400, lots of fuel filtration and a mechanical fuel pump (along with the electric pump in tank) It had a modified trans with a forged imput, and some other stuff)
Anywise, I was happy with the mileage. I usually got around 15-16 around town and 23 on freeway. I had a shell on the back.
Well, I pretty much totaled that truck (with no insurance). The back end of truck is great but the front in bent in up to the left front window piller.
I'm looking to either find a 94-98 single cab (no double cabs for me) and pump that up, or take my common rail engine and trans and drop it in. I'm not a mechanic, but I do almost everything on the truck. I will probably not be putting this engine in.
On the other hand, what are some common mileage numbers for a pumped up 94-98 with the p pump?
I'd want at least 500 hp. What does the typical 500 hp 2nd generation truck make mileage-wise?
Do the injectors make a huge difference in mileage? I also heard the the 2nd generation engines are easier to change injectors than my 3rd generation. I can do it, I've done it twice, but I heard the 2nd generation were much easier.